The semiconductor industry has recently experienced technological advances that have permitted dramatic increases in integrated circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-die microprocessors with many millions of transistors, operating at speeds of hundreds of millions of instructions per second to be packaged in relatively small, air-cooled semiconductor device packages.
A by-product of such high-density and high functionality is an increased demand for products employing these microprocessors and devices for use in numerous applications. As the use of these devices has become more prevalent, the demand for faster operation and better reliability has increased. Such devices often require manufacturing processes that are highly complex and expensive.
As the manufacturing processes for semiconductor devices and integrated circuits increase in difficulty, methods for testing and debugging these devices become increasingly important. Not only is it important to ensure that an individual die is functional, it is also important to ensure that batches of dice perform consistently. In addition, the ability to detect a defective manufacturing process early is helpful for reducing the number of defective devices manufactured.
Traditionally, integrated circuits have been tested using methods including directly accessing circuitry or devices within the integrated circuit. Directly accessing the circuitry is difficult for several reasons. For instance, in flip-chip type dice, transistors and other circuitry are located in a very thin epitaxially grown silicon layer in a circuit side of the die. The circuit side of the die is arranged face-down on a package substrate. This orientation provides many operational advantages. However, due to the face-down orientation of the circuit side of the die, the transistors and other circuitry near the circuit side are not readily accessible for testing, modification, or other purposes. Therefore, access to the transistors and circuitry near the circuit side is from the back side of the die.
One integrated circuit analysis method for conventional and flip-chip type dies involves using a liquid crystal material for detecting defects from the front side of an integrated circuit. Liquid crystalline materials have both crystalline solid and liquid characteristics. These characteristics enable their use for thermally analyzing an integrated circuit for defects. When the liquid crystal material is heated, its properties change. One such example change is a color change, and another change is an ordering transition. Available defect analysis methods use the change as an indication of temperature in an integrated circuit, which is useful for detecting defects that cause heat generation. By forming a liquid crystal layer on an integrated circuit, the response of the liquid crystal can be monitored and used to detect hot spots that are an indication of a defect.
One type of liquid crystalline material useful for front side defect analysis is calamatic liquid crystal material having nematic ordering. Calamatic liquid crystals have long, rod-shaped molecules, and those having nematic ordering change under temperature variation from a nematic to an isotropic state. In the nematic state, the liquid crystal alters the polarization of light incident upon it. When the liquid crystal changes to an isotropic state, the polarization of incident light is no longer affected. This change in the effect upon incident light is used to detect a temperature change in the liquid crystal material. The transition temperature at which the change occurs is dependent upon the particular characteristics of the material.
Typical analysis methods that use liquid crystals involve forming a liquid crystal layer on an integrated circuit and observing a change in the state of the liquid crystal that results from heat generated in the die. The liquid crystal layer is often formed by adding a solvent, such as pentane, to the liquid crystal material and then applying the material to the surface of an integrated circuit device with an eyedropper. The solvent evaporates, leaving the liquid crystal material behind. Other liquid crystal application methods include applying liquid crystal with a spreading strip, and applying a drop of liquid crystal on the chip and spinning the chip to spread out the liquid crystal. In addition, a liquid crystal emulsion may be used in place of the liquid crystal mixed with a solvent.
Once the liquid crystal has been applied, the integrated circuit is then heated with an external heater. The heater is used to bring the integrated circuit close to the transition temperature of the liquid crystal material. A microscope is directed at the liquid crystal layer. A suitable microscope includes a polarized light source and a linear polarizer (analyzer) in front of an eyepiece or camera. The integrated circuit is electrically stimulated, thereby heating a defect in the circuit and raising the liquid crystal material over the defect to its transition temperature. The liquid crystal material changes from nematic to isotropic phase, which is evidenced by a dark spot that is detected by the microscope.
In many applications, heating the defect via electrical stimulation causes a phase change that occurs too quickly to easily detect. In addition, heat from intrinsic heat sources in the die can overwhelm heat generated by a defect, making such analysis difficult.